1. Field of the Invention
The present invention relates to papermaking, and, more particularly, to fabric belts used in papermaking. Specifically, the present fabric belts are of the variety used to mold fibers into a three-dimensional structure, and, when so used, reduce non-uniform fiber distribution, pinholes and other irregularities frequently observed during such manufacturing processes.
2. Description of the Prior Art
Cellulosic fiber structures, such as newspaper, cardboard boxes, paper towels, facial tissues and toilet tissues, are a staple of contemporary life. The large demand for and constant use of such consumer products has created a need for improved versions thereof, and for improvements in their methods of manufacture. Such cellulosic fiber structures are manufactured by depositing an aqueous slurry from a headbox onto a Fourdrinier wire or between the wires on a twin wire paper machine. In either case, the forming wire is an endless fabric belt through which initial dewatering occurs and on which fiber rearrangement takes place. Frequently, fiber loss occurs when fibers flow through the forming wire along with the liquid carrier from the headbox.
After the initial formation of the web, which later becomes the cellulosic fiber structure, the web is transported to the dry end of the machine. In the wet end of a conventional machine, a press felt compacts the web into a single region cellulosic fiber structure prior to final drying. The final drying is usually accomplished by a heated drum, such as a Yankee drying drum.
In an improved manufacturing method, which yields corresponding improvements in the consumer products being manufactured, through-air drying replaces conventional press felt dewatering. In through-air drying, as in press felt dewatering, the web is initially formed on a forming wire which receives an aqueous slurry of less than one percent consistency (that is, the weight percent of fibers in the slurry is less than one percent) from a headbox. Initial dewatering takes place on the forming wire, but the web usually does not attain a consistency greater than 30 percent on the wire. From the forming wire, the web is transferred to an air-pervious through-air-drying belt.
Air passes through the web and through the through-air drying belt to continue the dewatering process. The air is driven by vacuum transfer slots, other vacuum boxes or shoes, predryer rolls, and other components. This air molds the web to the topography of the through-air-drying belt and increases the consistency of the web. This molding creates a more three-dimensional web, but also causes pinholes when the fibers in the web are deflected so far in a direction perpendicular to the plane of the through-air-drying belt that a breach in fiber continuity occurs.
After the web is molded on the through-air-drying belt, it is transported to the final drying stage, where it may also be imprinted. At the final drying stage, the through-air-drying belt transfers the web to a heated drum, such as a Yankee drying drum, for final drying. During this transfer, portions of the web may be densified in a specific pattern by imprinting to yield a multi-region structure. Paper products having such multi-region structures have been widely accepted by consumers. An early through-air-drying belt, which created a multi-region structure in the web by imprinting the knuckle pattern of its woven structure thereon, is shown in U.S. Pat. No. 3,301,746.
A subsequent improvement in through-air-drying belts was the inclusion of a resinous framework on the woven structure of the belt. Through-air-drying belts of this type may impart continuous or discontinuous patterns in any desired form, rather than knuckle patterns, onto the web during imprinting. Through-air-drying belts of this type are shown in U.S. Pat. Nos. 4,514,345; 4,528,239; 4,529,480; and 4,637,859.
The woven structure and the resinous framework of through-air-drying belts of this type provide mutual reinforcement for each other. The woven structure also controls the deflection of the papermaking fibers which results from vacuum applied to the backside of the belt and airflow through the belt. In early belts of this type, the woven structure was of a single-layer fine mesh, typically having approximately fifty machine-direction and fifty cross-machine-direction yarns per inch. While such a fine mesh was acceptable from the standpoint of controlling fiber deflection into the belt, it could not stand up to the environment of a typical papermaking machine for several reasons. One reason was that the fine mesh was so flexible that destructive folds and creases often occurred. In addition, the fine yarns did not provide adequate seam strength, and would often burn at the high temperatures encountered in papermaking.
Through-air-drying fabrics for the most part have been flat-woven, and subsequently joined into endless form with a woven seam. In general, there is a trade-off in flat-woven fabrics between seam strength and stretch resistance. This trade-off is controlled by the crimp in the warp yarns, which become the machine-direction yarns in a flat-woven fabric. In through-air-drying belts, which have a high open area (HOA), the trade-off is quite sensitive. In other words, as warp crimp is reduced to provide a fabric with more stretch resistance, seam strength will suffer, and vice versa. The balance between seam strength and stretch resistance is even more sensitive in an HOA fabric than in a more densely woven fabric, because there are relatively fewer warp yarns per unit of width in such a fabric.
Another problem, particularly encountered in tissue making, is the formation of small pinholes in the deflected areas of the web. It has recently been learned that pinholes are strongly related to the weave configuration of the woven structure in a through-air-drying belt.
A woven structure recently used for through-air-drying fabrics is a dual layer design having vertically stacked warps. A single weft yarn system ties the vertically stacked warps together. Generally, the conventional wisdom has been to use relatively large diameter yarns to increase fabric life. Fabric life is important not only because of their cost, but more importantly because of the expensive downtime incurred when a worn fabric must be removed from a papermachine and a new one installed. Larger diameter yarns, while being more durable, require larger holes between each other to accommodate the weave. The larger holes permit short fibers, such as those of Eucalyptus, to be pulled through the fabric and thereby create pinholes. Products made with such short fibers are heavily preferred by consumers because of the softness the short fibers impart to a cellulosic fiber structure.
This problem can be solved by weaving more yarns per inch into the pattern. However, this approach reduces the open area available for air flow. If yarns of smaller diameter are used to reopen the open area, the flexural rigidity and integrity of the woven structure of the belt are compromised and the fabric life is thereby reduced. Accordingly, the prior art also required a trade-off between the necessary open area (for airflow) and fiber diameter (for pinholing and belt life).
One attempt to achieve both good fiber support, and the flexural rigidity and belt integrity necessary to achieve a viable belt life, was to use a combination of large and small machine-direction (warp) yarns. The large diameter yarns provide the fabric with durability, and the smaller diameter machine-direction (warp) yarns are stacked above them on the web-facing layer for fiber support and pinhole reduction. An additional smaller diameter machine-direction (warp) yarn was placed on the paper-supporting side of the fabric between each stacked pair of machine-direction (warp) yarns for added fiber support. This attempt still did not satisfactorily reduce the occurrence of pinholes because the woven structure lacked planarity in that the additional machine-direction yarns were not supported from below by another yarn and tended to sag. The sagging leads to an increase in pinholing in the paper product being manufactured. In addition, the cross-machine-direction (weft) yarns which tied the two layers together went from the top of the paper-supporting layer to the bottom of the machine-contacting layer. This caused a further deviation from planarity which also contributed to increased pinholing.
The solution to these problems is one which recognizes that pinholing in a through-air-drying belt and fiber loss in a forming wire are related to the yarns that support the fibers, rather than to the open spaces between the yarns. The web-facing yarns must remain close to the top plane of the paper-supporting layer to provide adequate fiber support. In addition, the weave pattern must accommodate large diameter yarns in order to provide adequate fabric life.
Accordingly, it is an object of the present invention to provide a forming wire and a through-air-drying fabric which reduce non-uniform fiber distribution and pinholes in the product being manufactured. It is also an object of the present invention to provide a forming wire and a through-air-drying fabric in which the trade-off between seam strength and stretch resistance is balanced.